Surface-treated cathode active material and lithium secondary battery using the same

ABSTRACT

A surface-treated cathode active material useful for manufacturing a lithium secondary battery have excellent output characteristics by performing a double coating with metal oxide and an electron and ion conductive polymerized copolymer on a surface of a cathode active material for a lithium secondary battery to enhance electrochemical properties and thermal stability of the cathode active material.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of priorityto Korean Patent Application No. 10-2013-0167772 filed on Dec. 30, 2013,the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present disclosure relates to a surface-treated cathode activematerial and a lithium secondary battery using the same. Moreparticularly, the present disclosure relates to a surface-treatedcathode active material useful for manufacturing a lithium secondarybattery having excellent output characteristics by performing a doublecoating by primarily coating metal oxide on a surface of a cathodeactive material for the lithium secondary battery and secondarilycoating a conductive polymerized copolymer having both ion conductivityand electron conductivity thereon to enhance electrochemical propertiesand thermal stability of the cathode active material, and the lithiumsecondary battery using the same.

(b) Background Art

A lithium secondary battery is manufactured by using a material capableof intercalating and deintercalating lithium ions as a negativeelectrode and a positive electrode and intercalating an organicelectrolytic solution or a polymer electrolyte which enables lithiumions to move between the negative electrode and the positive electrode,and stores electrical energy by means of redox reactions according tothe intercalation and deintercalation of lithium ions in the positiveelectrode and the negative electrode.

In order to enhance electrochemical properties and safety of the lithiumsecondary battery as described above, studies on surface treatments ofthe cathode active material of the lithium secondary battery have beenactively performed.

In a case of the cathode active material for the lithium secondarybattery, dissolution of Li by HF produced in the electrolytic solutionmay be prevented by using metal oxide (Al₂O₃, ZrO₂ and La₂O₃), metalphosphorus oxide (AlPO₄ and LiCoPO₄), carbon, halogen gas, metalhydroxide, a conductive polymer and the like to reduce a direct reactionwith an electrolyte through a positive electrode surface coating ofseveral nm, and the stability of a crystal structure may be secured bysuppressing elution of various transition metals.

However, there is a limitation in that a drawback occurs, when themovement of lithium ions and electronic conduction are interrupted inthe case where a coating material, such as metal oxide which is anon-conductor, is used so that the mobility of lithium ions andelectrons deteriorates. Further, when coating with an inorganic particlealone, it is difficult to secure uniformity of the coating. Inparticular, it is insufficient to secure the structural stability of thecathode active material due to non-uniformity of the coating while aside reaction with the electrolytic solution occurs.

In a case of a polymer coating, it is possible to perform coating withan organic material as the related art in terms of securing theuniformity of the coating, however due to thermal instability of theorganic material and a too large thickness of the coating layer, ionsmay not be smoothly transfer, so that performance deteriorates.

Accordingly, depending on the particular inorganic material or polymer,the cathode active material has a limitation which may not be overcomein improving physical properties by only the surface modification.

For this reason, as a cathode active material surface modificationtechnology for improving high capacity, high output, and service lifecharacteristics of the lithium secondary battery, a technology ofcoating manganese oxide with a conductive polymer or obtaining a coatingby mixing two materials have been studied, however there is only anintention to obtain improvement in performance by simply increasing thecontent of a conductive material.

As an example of the related art, Korean Patent Application PublicationNo. 2007-8115 proposes a cathode active material sequentially includinga first covering layer of oxide on a surface of a lithium transitionmetal oxide particle, and a second covering layer of a conductivematerial on the first covering layer as a technology having a doublecoating structure. However, the material used in the second coveringlayer is a pure electron conductive material, and thus the movement oflithium ions is not smooth during a charging and discharging process.

Korean Patent Application Publication No. 2011-23067 proposes a cathodeactive material for a lithium secondary battery including a lithiummetal oxide secondary particle core, a first shell formed by coating thesurface of a secondary particle core portion with a plurality of bariumtitanate particles and a plurality of metal oxide particles, and asecond shell formed by coating a surface of the first shell with aplurality of olivine-type lithium iron phosphate oxide particles and aplurality of conductive material particles. Furthermore, Korean PatentApplication Publication No. 2007-16431 proposes an active material for alithium secondary battery, which has a core material and a surfacetreatment layer which is formed on a surface of a core material andincludes an inorganic particulate having a nano-size and conductivepolymer.

However, according to the technologies of the surface coating-treatedcathode active material as described above, although surfacemodification effects are improved as compared to the existingtechnologies, conductivity and ion transfer effects are not fullyexhibited, and effects of the improvement in performance such aselectrochemical properties and thermal stability are slight.

Japanese Patent Application Publication No. 2005-524936 proposes anelectrode manufactured while a conductive material and an ion conductingpolymer are extruded into an active material including metal oxidethrough an extruder. However, effects of the improvement in performancesuch as electrochemical properties and thermal stability are not verygood because the structure of the active material is unstable andnon-uniform.

In addition to the technologies described above, there are severaltechnologies using a conductive polymer and a metal powder, such as atechnology in which a cathode active material is coated with a polymerhaving both ion conductivity and electric conductivity and a polymerfilm to which a conductive metal powder is added. However, it does notsufficiently resolve the existing problems in improving physicalproperties, such as maintaining a balance between conductivity and iontransfer effects.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention, andtherefore, it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE DISCLOSURE

The present disclosure has been made in an effort to solve theabove-described problems associated with the related art.

As a result of intensive studies for solving the problems in the relatedart, it has been found that when a cathode active material for a lithiumsecondary battery is double-coated by primarily coating an inorganicmaterial on a surface of the cathode active material, and secondarilycoating a conductive polymerized copolymer having both electronconductivity and ion conductivity thereon, electrochemical propertiesand thermal stability of the cathode active material are greatlyenhanced. As a result, a lithium secondary battery having excellentoutput characteristics may be manufactured.

An aspect of the present disclosure provides a cathode active materialdouble-coated with metal oxide and a polymerized copolymer havingelectron conductivity and ion conductivity on a surface of the cathodeactive material.

Another aspect of the present disclosure provides a cathode activematerial with a surface modified such that electrochemical propertiesand thermal stability are greatly enhanced.

Still another aspect of the present disclosure provides a lithiumsecondary battery having a long service life and excellent outputcharacteristics by using a cathode active material with electrochemicalproperties and thermal stability that are greatly enhanced due to asurface modification.

According to an exemplary embodiment of the present disclosure, asurface-treated cathode active material is double-coated by primarilycoating metal oxide on a surface of a cathode active material, andsecondarily coating a polymerized copolymer having both electronconductivity and ion conductivity thereby forming a double coating onthe surface of the cathode active material.

In another aspect of the present disclosure, a lithium secondary batterycomprising the surface-treated cathode active material is provided.

The surface-treated cathode active material according to the presentdisclosure is a cathode active material in which electrochemicalproperties of a positive electrode material for a lithium secondarybattery are greatly enhanced by a double coating treatment with metaloxide and an electron and ion-conductive copolymerized polymer.

In particular, it is possible to enhance high voltage and service lifecharacteristics as well as structural and thermal stability throughsurface modification of the cathode active material by secondarily anduniformly coating a polymer having both electron conductivity and ionconductivity on a primarily coated inorganic material as compared toelectrode materials coated with only the inorganic material.

It is also possible to manufacture a lithium secondary battery havingbetter output characteristics than existing electrodes coated with aninorganic material by using a polymer material having electronconductivity and ion conductivity to enhance the movement of lithiumions and electron conductivity of the electrode.

Other aspects and embodiments of the invention are discussed infra.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general; such aspassenger automobiles, including sports utility vehicles (SUV); buses;trucks; various commercial vehicles; watercraft, including a variety ofboats and ships; aircraft; and the like; and includes hybrid vehicles;electric vehicles; plug-in hybrid electric vehicles; hydrogen-poweredvehicles; and other alternative fuel vehicles (e.g., fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example, bothgasoline-powered and electric-powered vehicles.

The above and other features of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present disclosure will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated in the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present disclosure.

FIG. 1 is a schematic view illustrating a structure in which asurface-treated cathode active material according to the presentdisclosure is double-coated.

FIG. 2 is a concept view of a coating process schematically illustratinga double coating process of the surface-treated cathode active materialaccording to the present disclosure.

FIGS. 3( a)-3(d) are photographs comparing differently coated cathodeactive materials including a scanning electron microscope (SEM)photograph of LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ double-coated with aninorganic material and a conductive polymer, and an SEM photograph ofpristine LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂.

FIGS. 4( a)-4(d) are transmission electron microscope (TEM) comparisonphotographs of LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ before and after beingcoated with each component according to the present disclosure.

FIGS. 5A and 5B are graphs comparing cycle characteristics of anLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ electrode according to a charging voltagefor the cathode active material, in which polyethylenedioxythiophene(PEDOT) or polyethylenedioxythiophene polyethylene glycol (PEDOT-PEG)are applied as coating materials and compared with each other. In FIG.5A, the charging voltage is 4.3 V and in FIG. 5B, the charging voltageis 4.6 V.

FIG. 6 is a graph comparing cycle characteristics for each thickness ofa double coating by double coating the cathode active material, wherecharging and discharging are performed 50 times at a charging voltage of4.6 V.

FIGS. 7A and 7B are graphs comparing cycle characteristics of theLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ electrode according to the charging voltagefor the cathode active material, in which Al₂O₃ or Al₂O₃+PEDOT-PEGdouble coating are applied as coating materials and are compared witheach other. In FIG. 7A, the charging voltage is 4.3 V and in FIG. 7B,charging voltage is 4.6 V.

FIG. 8 is a graph comparing enhancements in cycle characteristics of theLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ electrode at a high temperature (55° C.)according to the coating of the cathode active material.

FIG. 9 is a graph comparing discharging capacities of theLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ electrode according to a discharging rate(C rate) of the cathode active material.

FIG. 10 is a graph comparing enhancements in thermal stability of theLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ electrode according to the coating of thecathode active material.

FIGS. 11A and 11B are graphs comparing cycle characteristics of alithium secondary battery according to a charging voltage for thelithium secondary battery to which the cathode active material isapplied. In FIG. 11A, the charging voltage is 4.3 V and in FIG. 11B, thecharging voltage is 4.6 V.

FIG. 12 is a graph comparing cycle characteristics of a lithiumsecondary battery at a high temperature (55° C.) for the lithiumsecondary battery in which the cathode active material is used.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawings.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present disclosure, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents, and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

Hereinafter, an exemplary embodiment of the present disclosure will bedescribed in more detail as follows.

The present disclosure relates to a surface-treated cathode activematerial in which metal oxide and a polymerized copolymer having bothelectron conductivity and ion conductivity are successivelydouble-coated on a surface of the cathode active material.

According to the present disclosure, as the cathode active material,those selected from lithium transition metal oxides and sulfur compoundsmay be used. Specific examples of the cathode active material which maybe used in the present disclosure include LiCoO₂, LiNi_(x)Co_(y)Mn_(z)O₂(0≦x≦1, 0≦y≦1, 0≦1≦z), LiNi_(0.5)Mn_(1.5)O₄, LiMn₂O₄, LiFePO₄, sulfur,and the like.

As an example of the metal oxide which is primarily coated on thesurface of the cathode active material in the present disclosure, it ispossible to use one or more selected from the group consisting of Al₂O₃,SiO₂, TiO₂, SnO₂, CeO₂, ZrO₂, BaTiO₃, and Y₂O₃. As the metal oxide,those having an average particle diameter of 1 nm to 100 nm may be used.

According to the present disclosure, the metal oxide may be coated in anamount up to 0.1 to 2.0 wt % based on the weight of the cathode active.

According to the present disclosure, the metal oxide is primarily coatedin a small thickness on the surface of the cathode active material, andthen a polymerized copolymer having both electron conductivity and ionconductivity is coated as a secondary coating thereon.

As an example of the electron conductive polymer used herein, it ispossible to use one or more selected from the group consisting ofpolythiophene, polyethylene dioxythiophene, polyaniline, polypyrrole,and polyacetylene.

As an example of the ion conductive polymer, it is possible to use oneor more selected from the group consisting of polyethylene glycol,polypropylene glycol, polyalkylene carbonate, and polyester.

According to the present disclosure, as the polymerized copolymer havingboth electron conductivity and ion conductivity, the copolymer of theaforementioned electron conductive polymer, and the aforementioned ionconductive polymer may be used. As the polymerized copolymer, poly(3,4-ethylenedioxythiophene)-block-poly (ethylene glycol) (PEDOT-PEG)may be used.

According to the present disclosure, the cathode active materialsuccessively double-coated with the metal oxide and the polymerizedcopolymer as described above may have a total coating thickness of 5 to500 nm. In certain embodiments, the total coating thickness may be 10 to100 nm. When the coating layer is too thin, it is difficult to expectenhancement in performance, and when the coating layer is too thick,resistance encountered in transfer of ions and electrons is increased,so that it is difficult to obtain a high capacity.

FIG. 1 is a schematic view illustrating a structure in which asurface-treated cathode active material according to the presentdisclosure is double-coated.

According to the present disclosure, drawbacks of each of the inorganicmaterial and the polymer may be compensated for by a double coatingmethod using both inorganic material and polymer instead of using theinorganic material or the polymer alone.

First, a metal oxide, which is the inorganic material applied as aprimary coating of the cathode active material, is primarily coated onthe surface of the cathode active material in order to secure thermalstability of the metal oxide and improve service life characteristics. Apolymerized copolymer having both electron conductivity and ionconductivity is subsequently secondarily coated in order to overcomenon-uniformity of the primarily coated inorganic material andsimultaneously enhance electron conductivity and ion conductivity of thecathode active material. In particular, the polymerized copolymer havingboth electron conductivity and ion conductivity significantly improvesthe reduction in electron and ion conductivities caused by the coatingwith the primary metal oxide, which is a non-conductor. When thepolymerized copolymer having electron conductivity and ion conductivityis coated on the cathode active material, smoother ion transfer andelectron conduction are exhibited in intercalation and deintercalationreactions of lithium than that provided by other polymers, and thus,capacity and output characteristics are greatly enhanced.

When the cathode active material according to the present disclosure,which is surface-treated with the polymerized copolymer having electronconductivity and ion conductivity used in the present disclosure, suchas a PEDOT-PEG polymer, is used in a lithium secondary battery, acoating layer may be stably maintained during charging and dischargingcycles because the coating layer does not dissolve in ethylene carbonate(EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethylcarbonate (DEC), and the like, which are used in an electrolyticsolution for the lithium secondary battery. Accordingly, structuralstability of the cathode active material may be maintained, and a sidereaction with the electrolytic solution may be reduced. In particular,the effects of the coating at high voltage and high temperature, atwhich decomposition of the electrolytic solution occurs, are maximized.Furthermore, since the polymer is used as a final coating layer, thepresent disclosure has an additional effect in that adhesion strengthwith a polymer material used as a binder for the lithium secondarybattery is increased. The effect according to the present disclosure isdemonstrated in the following Examples.

As described above, the present disclosure may enhance electrochemicalcharacteristics and thermal stability of the cathode active material fora lithium secondary battery by double coating the metal oxide coatingand the electron and ion conductive polymerized copolymer. Inparticular, it is possible to enhance high voltage and service lifecharacteristics as well as structural and thermal stability by modifyingthe surface by primarily and thinly coating the metal oxide on thesurface of the cathode active material, and secondarily and uniformlycoating a polymer having both electron conductivity and ion conductivityon the primary coating layer as compared to electrode materials coatedwith only the inorganic material.

According to the present disclosure, it is possible to provide a lithiumsecondary battery having better output characteristics than a batteryusing an existing electrode coated with an inorganic material by using apolymer material having electron conductivity and ion conductivity toenhance the movement of lithium ions and electron conductivity of theelectrode as described above.

Accordingly, the present disclosure provides a lithium secondary batteryincluding the surface-treated cathode active material as describedabove.

The lithium secondary battery manufactured by using the surface-treatedcathode active material according to the present disclosure hasexcellent output characteristics and improvement in long service lifecompared to existing lithium secondary batteries.

The lithium secondary battery manufactured by using the surface-treatedcathode active material according to the present disclosure may beapplied not only to small secondary batteries used in mobile electronicdevices, such as mobile phones, laptop computers, and digital cameras,but also to medium to large energy storing devices, and the like, whichare used in electric vehicles, energy storing systems, and the like. Inparticular, the lithium secondary battery may be applied to an electricvehicle, which consumes a relatively high amount of energy and isexcessively used over a long period of time.

Hereinafter, the present disclosure will be described in detail withreference to Examples, but is not limited by the Examples.

Examples

The following examples illustrate the invention and are not intended tolimit the same.

Example 1 Surface Treatment of Cathode Active Material

A schematic surface coating process of an active material is illustratedin FIG. 2. Coating was performed as illustrated in FIG. 2. A cathodeactive material may be applied to all metal oxide materials, and wasapplied to the Li[Ni_(0.6)Co_(0.2)Mn_(0.2)]O₂ (hereinafter, NMC 622)material in the present Example. A dry coating was performed on the NMC622 surface using aluminum oxide (Al₂O₃, Aldrich) having a size of 2 to9 nm, and herein, an amount of 0.5 wt % was coated. For the dry coating,a ball-milling method known in the art was used, and dry coating wasperformed at a speed of 300 rpm for 6 hours.

The cathode active material subjected to dry coating was introduced intoa nitromethane solution in which a PEDOT-PEG polymer was dissolved, andthe resulting solution was stirred at temperature of 60° C. for 6 hours.After stirring, a solid was obtained through a filtering process, andthe double coating on the surface of the cathode active material wascompleted by vacuum-drying the solid at 110° C.

Example 2 Morphology Analysis of Cathode Active Material

Morphology of the cathode active material subjected to coating wasconfirmed. In order to confirm the degree of dispersion of the coatingmaterial on the surface of the electrode subjected to coating, mappingresults of various elements were analyzed using a scanning electronmicroscope and illustrated in FIGS. 3( a)-3(d).

As a result of comparison with the cathode active material (pristinepositive electrode) which was not coated, aluminum (Al) and sulfur (S)contained in the inorganic material and the conductive polymer weredetected in the case of the double coated cathode active material, andit could be seen that these elements were uniformly coated on thesurface of the active material. The photographs observed using TEM inFIGS. 4( a)-4(d) each shows (a) the surface of the active materialbefore coating, (b) the surface of the active material after coatingwith the electron and ion conductive polymer, (c) the surface of theactive material after primary coating with metal oxide, and (d) thesurface of the active material subjected to double coating with theinorganic material and the electron and ion conductive polymer. Herein,it can be seen that uniformity of the coating layer may be secured bysecondarily coating the conductive polymer on the surface of theprimarily coated inorganic material.

Example 3 Manufacture of Electrode and Cell Using the Coated CathodeActive Material

After poly (vinylidene fluoride) (PVdF), used as a binder in order tomanufacture a positive electrode; was completely dissolved inN-methylpyrrolidone, super-P carbon, as a conductive material; anddouble-coated LiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ were measured and put intothe resulting mixture solution, and the resulting mixture was stirred.In this case, a weight ratio of the cathode active material, theconductive material, and the binder was set to 85:7.5:7.5.

The completely mixed slurry solution was applied on an aluminum foil,dried, and was subjected to a lamination process using a roll press.This was performed in order to enhance a mutual bonding force of theactive material/the conductive material/the binder and effectively bindthe materials to the current collector.

When the compression process was finished, an electrode having asuitable size was manufactured through a cutting process, and dried at110° C. in a vacuum oven for 24 hours. A coin cell was manufacturedusing the manufactured positive electrode.

Lithium metal was laminated on a copper foil, and the resulting laminatewas used as a negative electrode. A solution, prepared by dissolving 1 MLiPF₆ in a mixed solvent of ethylene carbonate/dimethyl carbonate(volume ratio 50/50), was used as an electrolytic solution, and apolyethylene separation film was used as a separation film. Thepreparation of all the electrodes was performed in a dry room, and themanufacture of a battery was performed in a glove box in which an argonatmosphere was maintained.

For the manufactured cell, charging and discharging cycles wereperformed in a range of 2.6 to 4.3 V or 2.6 to 4.6 V with a currentdensity of 0.5 C.

Example 4 Evaluation of Characteristics of Lithium Secondary Battery

Service life characteristics of cells subjected to a formation processwere evaluated for each charging voltage (4.3 V and 4.6 V) using each ofa positive electrode (pristine) which was not coated, a positiveelectrode (modified NCM PEDOT) coated with only an electron conductivepolymer, and a positive electrode (modified NCM PEDOT-PEG) coated with apolymer having both ion conductivity and electron conductivity. Chargingand discharging results as illustrated in FIGS. 5A and 5B were obtainedby repeating the charging and discharging 50 times with a currentdensity of 0.5 C at room temperature. As a result, it can be seen thatin the case of coating with the PEDOT-PEG, which is an ion conductiveand electron conductive copolymerized polymer, the highest capacity andexcellent service life characteristics are obtained.

The cathode active material was double coated with primarily aluminumoxide and secondarily a PEDOT-PEG copolymer having electron conductivityand ion conductivity, and cycle characteristics for each coatingthickness were evaluated and illustrated in FIG. 6. As a result, whenthe coating thickness was as thin as 8 nm, it can be seen that theinitial capacity was high, but the service life characteristics rapidlydeteriorated. When the coating thickness was as thick as 180 nm, thecoating layer served as a resistor, so that a low discharging capacityvalue was obtained in the initial stage. In contrast, a positiveelectrode material double coated within a thickness of 30 nm showedexcellent service life characteristics along with a high dischargingcapacity in an initial stage.

The service life characteristics were evaluated for each chargingvoltage (4.3 V and 4.6 V) in a case where the cathode active materialwas coated with only aluminum oxide and a case where the cathode activematerial was double coated with the aluminum oxide and the polymerhaving electron conductivity and ion conductivity. Results, asillustrated in FIGS. 7A and 7B, were obtained by repeating the chargingand discharging 50 times with a current density of 0.5 C at roomtemperature.

Referring to FIGS. 7A and 7B, the two cathode active materials coatedwith the aluminum oxide showed enhanced service life characteristics atboth (FIG. 7A) 4.3 V and (FIG. 7B) 4.6 V, compared to the cathode activematerial which was not coated. However, a side reaction with a partialelectrolytic solution was caused by non-uniformity of the coating.Therefore, the double coating compensated for the coating with theinorganic oxide material layer still has some drawbacks. The problem ofthe coating layer was overcome by introducing an electron and ionconductive copolymerized polymer. The double coated cathode activematerial showed the best performance and service life characteristics,and it was confirmed that the double coated cathode active materialshowed an excellent performance even when charging voltages weredifferent.

Service life characteristics were compared in a high temperaturecondition of 55° C., and results are illustrated in FIG. 8. Referring toFIG. 8, since cell internal resistance is decreased at high temperature,it can be seen that a higher capacity is implemented at high temperaturethan that at room temperature (see FIG. 7A). In contrast, it can be seenthat the positive electrode material which is not coated has relativelypoor service life characteristics while structural collapse of thecathode active material and the side reaction with the electrolyticsolution also further rapidly occur. However, it can be seen that anelectrode double coated with aluminum oxide and a conductive polymershows high capacity and excellent service life characteristics. This isbecause the double coated metal oxide and polymer coating layereffectively suppresses the side reaction with the electrolytic solution.

Example 5 Evaluation of High Rate Characteristics

The charging and discharging evaluation was performed three timesaccording to each current density while increasing the current densityfrom low rate to high rate. This is a test in which capacitycharacteristics of the cathode active material are confirmed underconditions where the rapid charging and discharging is performed and atechnique which evaluates characteristics by successively applying highcurrent, unlike the evaluation of service life characteristics, in whicha constant current density is applied.

Results of evaluating the charging capacity are illustrated in FIG. 9.The results in FIG. 9 show that the double coated electrode exhibitshigh capacity at a high rate which requires a high output. Inparticular, a capacity recovery rate of the cathode active material isconfirmed by changing the current density from a low rate to the highrate, and the structural stability may be indirectly confirmed. For thedouble coated electrode as illustrated in FIG. 6, a high capacityrecovery rate of 99.8% was obtained.

Example 6 Evaluation of Thermal Stability after Evaluation of ServiceLife Characteristics

After the service life characteristics of the lithium secondary batterywere evaluated, the battery was dissembled in a charging state, thermalstability of the cathode active material was analyzed by a differentialscanning calorimeter (DSC) and illustrated in FIG. 10, and results aresummarized in the following Table 1.

In a case of the coated electrode, it can be seen that an exothermicpeak shifted to a high temperature region, and the amount of heatreleased was rapidly decreased. It can be seen that the coating layercomposed of the metal oxide and the polymer suppresses an exothermicreaction of the cathode active material with the electrolytic solution,so that thermal stability was enhanced.

The following Table 1 compares DSC thermal characteristics of thecathode active material in a charging state after charging anddischarging.

TABLE 1 Exothermic peak Amount of heat Electrode temperature (° C.)released (J/g) Pristine positive electrode 281.0 587.5 Aluminum oxidecoated positive 286.9 320.1 electrode Double coated positive electrode287.4 198.5

Example 7 Evaluation of Performance of Lithium Secondary Battery

A lithium secondary battery was manufactured by using a graphite-basedmesocarbon microbeads (MCMB) as a negative electrode, andcharacteristics for each condition were evaluated. After the lithiumsecondary battery was manufactured, service life characteristics ofcells formed in accordance with the present disclosure were evaluatedfor each charging voltage (4.3 V and 4.6 V). Charging and dischargingresults as illustrated in FIGS. 11A and 11B were obtained by repeatingcharging and discharging 50 times with a current density of 0.5 C atroom temperature. When the charging voltage was 4.3 V, the positiveelectrode double coated with aluminum oxide and a PEDOT-PEG polymerhaving electron conductivity and ion conductivity showed the highestinitial capacity and excellent service life characteristics asillustrated in FIG. 11A. FIG. 11B illustrates service lifecharacteristic results for the charging of high voltage (4.6 V). It canbe seen that as the charging voltage is increased, the service lifecharacteristics are significantly enhanced through a double coating ofaluminum oxide and PEDOT-PEG.

For a lithium secondary battery, in which the double coated cathodeactive material was used, cycle characteristics were evaluated at hightemperature and illustrated in FIG. 12. In the graph of FIG. 12, coatedactive materials show better service life characteristics at a hightemperature (55° C.) than a case in which uncoated cathode activematerial is used. In particular, due to suppressing destruction effectsof the crystal structure of the cathode active material along with adecomposition of the electrolytic solution, double coating with thealuminum oxide and the PEDOT-PEG polymer show enhanced service lifecharacteristics compared to the existing cathode active material.

The surface-treated cathode active material according to the presentdisclosure may be applied to small and medium to large lithium secondarybatteries. In particular, the surface-treated cathode active materialaccording to the present disclosure may be applied to lithium secondarybatteries used in various mobile electronics, electric vehicles, andenergy storing systems.

The invention has been described in detail with reference to theembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

What is claimed is:
 1. A surface-treated cathode active material whichis double-coated by primarily coating metal oxide on a surface of acathode active material and secondarily coating a polymerized copolymerhaving both electron conductivity and ion conductivity thereby forming adouble coating on the surface of the cathode active material.
 2. Thesurface-treated cathode active material of claim 1, wherein the cathodeactive material is selected from lithium transition metal oxides andsulfur compounds.
 3. The surface-treated cathode active material ofclaim 1, wherein the metal oxide is one or more selected from the groupconsisting of Al₂O₃, SiO₂, TiO₂, SnO₂, CeO₂, ZrO₂, BaTiO₃, and Y₂O₃. 4.The surface-treated cathode active material of claim 1, whereinparticles of the metal oxide have an average particle diameter of 1 to100 nm.
 5. The surface-treated cathode active material of claim 1,wherein the polymerized copolymer having both electron conductivity andion conductivity comprises one or more electron conductive polymersselected from the group consisting of polythiophene, polyethylenedioxythiophene, polyaniline, polypyrrole, and polyacetylene.
 6. Thesurface-treated cathode active material of claim 1, wherein thepolymerized copolymer having both electron conductivity and ionconductivity comprises one or more electron conductive polymers selectedfrom the group consisting of polyethylene glycol, polypropylene glycol,polyalkylene carbonate, and polyester.
 7. The surface-treated cathodeactive material of claim 1, wherein the polymerized copolymer havingboth electron conductivity and ion conductivity is a poly(3,4-ethylenedioxythiophene)-block-poly (ethylene glycol) (PEDOT-PEG).8. The surface-treated cathode active material of claim 1, wherein atotal thickness of double coating is 10 to 100 nm.
 9. A lithiumsecondary battery comprising the surface-treated cathode active materialof claim
 1. 10. The lithium secondary battery of claim 9, wherein thebattery is for a mobile electronic battery, an electric vehicle battery.11. A lithium secondary battery comprising the surface-treated cathodeactive material of claim
 2. 12. A lithium secondary battery comprisingthe surface-treated cathode active material of claim
 3. 13. A lithiumsecondary battery comprising the surface-treated cathode active materialof claim
 4. 14. A lithium secondary battery comprising thesurface-treated cathode active material of claim
 5. 15. A lithiumsecondary battery comprising the surface-treated cathode active materialof claim
 6. 16. A lithium secondary battery comprising thesurface-treated cathode active material of claim
 7. 17. A lithiumsecondary battery comprising the surface-treated cathode active materialof claim 8.